It is conventional in the prior art to provide locking valves that prevent exhaust flow from a hydraulic actuator and arrest the associated piston at the end of its stroke. The deficiency in these hydraulic stroke control valves is that leakage passed the main piston permits the piston to creep somewhat and tightly drive the exhaust control valve into its valve seat and thereby prevent flow from the pump to the extending side of the hydraulic actuator.
More particularly, the present invention relates to a hydraulic actuator having a piston and piston rod slidable therein with fluid interconnections to a directional control valve for driving the piston in opposite directions.
An exhaust flow blocking valve is provided for preventing exhaust flow from one side of the hydraulic actuator thereby arresting the piston at the end of its retracting stroke.
An actuating element is provided on the main piston rod for driving this valve to its closed position. As the piston nears its end of stroke the actuating element drives the valve towards its closed position and as it nears its closed position, fluid flow, exhausting from the cylinder, snaps the valve to its fully closed position, leaving a gap between the valve stem and the actuating element, which gap permits the valve to open upon reversal of the directional control valve. However, it has been found in many cases that fluid leakage past the main piston results in creepage of the piston and rod so that the actuating element engages the valve stem and eliminates any gap therebetween. Thus upon reversal of the directional control valve, the blocking valve is unable to open preventing reversal of the actuator and frequently damaging the actuating element.
It is a primary object of the present invention to overcome this prior art problem.